1. Field of the Invention
This invention relates generally to ceramics and particularly to a method of preparing ceramics that uses a self generated low melting flux.
2. Description of the Prior Art
Ceramic materials play an important role in modern technology. Taking ferrite materials as an example, they are used in such diverse applications as inductor cores for filters, high frequency transformer cores, magnetic memory cores, magnetic recording tapes, magnetic recording heads, and magnets.
The intended use of the ferrite dictates the ferrite microstructure and compositional properties needed to obtain the desired magnetic and mechanical properties. High density ferrites are often desired because the high density reduces the number of defects, such as voids or pores, which degrade magnetic properties such as permeability and mechanical properties such as strength. Large grain size is required for some applications, such as high permeability cores, where the large pore free grains maximize permeability. For other applications, such as low loss inductors, both high density and small grain size are desired to minimize losses.
Considerable effort has been expended in developing techniques to prepare ferrite materials with the desired composition and microstructure. A commonly used method involves solid state reactions, at high temperatures, between the oxides or carbonates of the ferrit cations. The method begins with very fine particles of, for example, oxides or carbonates and then thoroughly mixes them. The mixture is then calcined, milled, granulated, pressed, and sintered. While this method is perfectly adequate for many purposes, the high temperatures required for the sintering step are undesirable as relatively volatile material may be lost from the ceramic composition and consequently compositional uniformity within the ceramic part is difficult to control. Additionally, since lower sintering temperatures inevitably translate into lower production costs, other factors such as sintering time being constant, the high temperatures are an economical liability.
It is well known that the presence of a suitable flux may aid a sintering process, and permit it to proceed at temperatures lower than otherwise possible, by providing a medium or mechanism through which the materials react and diffuse more rapidly. The flux may be in the liquid phase but temperatures below the melting temperature of the flux are also useful. The solid state sintering rate increases with the homologous temperature, T/T.sub.m, where T is the absolute temperature and T.sub.m the melting temperature of the flux.
With ferrites, for example, fused salts have been used to provide such a flux. Ferrites. Proceedings of the International Conference Kyoto 1970 pp. 105-107. This approach has drawbacks. Provision must be made in the processing sequence for removal of the salts and the ferrite has relatively low solubility in the flux thus limiting the attainable grain size. The flux system BaO-Bi.sub.2 O.sub.3 -B.sub.2 O.sub.3 has been used to prepare nickel ferrite crystals. Ferrites. Proceedings of the International Conference Kyoto 1970 pp. 303-305. This approach has the drawback of putting undesired impurities, which can adversely alter the magnetic properties, into the ferrite composition.
The use of a low melting flux, formed from components of the desired final composition, would avoid the described drawbacks. Such fluxes have apparently been used in the preparation of metallic alloys, e.g., Co.sub.5 Sm, Journal of Applied Physics 41 pp. 5247-5249, December 1970. While it has been realized that low melting eutectics might be present in ceramics, e.g., lithia stabilized .beta."-Al.sub.2 O.sub.3 Ceramic Bulletin 56 pp. 206-212, 1977, it has not been realized that such low melting fluxes could be prepared and used in the preparation of ceramic materials such as ferrites and garnets.